Elsevier

Chemical Engineering Journal

Volume 239, 1 March 2014, Pages 149-157
Chemical Engineering Journal

Preparation, characterization and charge transfer studies of nickel – modified and nickel, nitrogen co-modified rutile titanium dioxide for photocatalytic application

https://doi.org/10.1016/j.cej.2013.11.016Get rights and content

Highlights

  • (Ni, N) co-modified rutile was prepared via impregnation–calcination method.

  • Ni-single modified and N-single modified rutile was prepared for comparison.

  • High UV and visible light activity of new materials confirmed by AS measurements.

  • Possible charge transfer between TiO2 and NiTiO3, TiN or metallic nickel species.

  • Studies of the influence of Ti3+ ions on photocatalytic activity of the new materials.

Abstract

A simple way of preparation of Ni-modified and Ni,N-co-modified rutile TiO2 from an industrial grade amorphous titanium dioxide via impregnation followed by calcination at 1073 K in argon (Ni–TiO2) or ammonia (Ni,N–TiO2) atmosphere with different Ni:TiO2 ratios is presented. New materials were characterized by means of: XRD, XPS, elemental analysis, UV–Vis/DR spectroscopy, SEM, EPR and N2 adsorption at 77 K. The photocatalytic activity of the materials was studied during decomposition of acetic acid under irradiation with UV or Vis light with wavelengths longer than 400 nm or 450 nm. The obtained results suggest the occurrence of a charge transfer between TiO2 and NiTiO3 phases resulting in a relatively high activity of XNi–TiO2 materials under the irradiation up to 400 nm. A band-gap narrowing of TiO2 after ammonia treatment was proved on the basis of UV–Vis/DR spectra for all the co-modified samples. The influence of Ti3+ ions on the band-gap narrowing and consequently photocatalytic activity of XNi,N–TiO2 and possible electron transfer from rutile conduction band to metallic Ni is here discussed.

Introduction

Removal of organic contaminants from air and water is a considerable problem which has to be solved in order to provide us access to drinkable water and pollution-free air zones. Titanium dioxide (TiO2)-based photocatalysis is considered to be one of the most effective ways for decomposition and mineralization of numerous pollutants. Nevertheless, optimization of the process of preparation of highly active and stable materials is still open to explore.

The three main difficulties to be overcome in order to obtain sufficient activity of TiO2 for commercial application are: enhancing its visible-light absorption, inhibiting charge-recombination process and improving stability of the material. Different modification methods have been employed in order to enhance the photocatalytic activity of TiO2 by either narrowing the band gap [1], mainly via doping with non-metals such as carbon [2], [3], sulfur [4] or nitrogen [5], [6] which may also induce oxygen vacancies incorporation [7], [8] or by inhibiting the electron–hole recombination [9] via e.g. modification with metals [10] or semiconductors coupling [11], [12], however, the results of these modifications seem to be not completely satisfying. Therefore, new methods of improving the TiO2 photocatalytic performance have to be developed.

A promising solution of enhancement of TiO2 activity could be a combination of two of the above mentioned methods, one improving the visible light absorption abilities and the other which could provide better electron/hole pairs separation. The most popular way of combining these kinds of modification is non-metal, metal co-modification, such as C, Ag-co-modification which has been studied by Liu et al. [13]. What they pointed out was that this kind of co-modification improved photocatalytic performance of TiO2 under both UV and visible light irradiation, resulting in higher activity than in case of unmodified and Ag-modified TiO2. Niu et al. [14] have prepared S, Fe co-modified photocatalysts via sol–gel process and low-temperature solvothermal method. The sulfur and/or iron modification caused band-gap narrowing, and Fe3+ cations doped into TiO2 lattice and SO42- adsorbed on the surface of TiO2 could trap photogenerated electrons, improving the separation of photogenerated electrons and holes. Nevertheless, still the most advantageous non-metal for TiO2 modification, chosen by many researchers, seems to be nitrogen. Thus, nitrogen, metal co-modification has also attracted a considerable attention. Li et al. [15] have prepared W,N-co-doped TiO2 via sol–gel method and reported its enhanced visible light absorption as well as artificial solar light activity. Furthermore, Jaiswal et al. [16] have studied V,N co-modification also via sol–gel method. They found that the visible light absorption of TiO2 increased after vanadium or nitrogen-single modification as well as co-modification processes, but the highest visible light activity was obtained in the case of the co-modified sample. On the top of this research Dan et al. [17] have performed theoretical calculations concerning Co,N-co-modified rutile TiO2 and stated that N-doped, Co-doped and N,Co-co-doped rutile TiO2 all extend the optical absorption in the visible-light region.

During our previous investigations [18] we have prepared series of Fe,N co-modified rutile photocatalysts. We proved their superior photoactivity under visible light irradiation compared to both commercial TiO2 P25 as well as pure rutile. Moreover, we confirmed and explained the synergistic effect of Fe,N co-modification. To the best of our knowledge there are no other reports available on the rutile modification via impregnation–calcination in ammonia atmosphere apart from the above mentioned paper by our group. In the present work, we have discussed the results of rutile co-modification with the same non-metal, i.e. nitrogen and another metal–nickel. We have selected Ni due to its promising properties, revealing from the literature reports. Recently, Iwaszuk et al. [19] reported that NiO-modified TiO2 P25 exhibited visible-light activity during degradation of 2-naphthol and p-cresol almost twice higher than that of FeOx/TiO2 P25. The improvement of visible light activity of Ni-modified titania was also reported by Zang et al. [20] and Murakami et al. [21]. Yoshinaga et al. [22] proved enhancement of visible light photoactivity of thin film amorphous titanium dioxide materials prepared by pulsed laser deposition method by their co-modification with sulfur and nickel. Recently, Cao et al. [23] published their results on photoactivity of nitrogen and nickel–chlorine co-modified anatase TiO2 photocatalysts during degradation of 4-chlorophenol. The authors found that the co-modification of N–Ox and O–Ni–Cl surface species led to the enhancement of visible-light response in comparison with both pure anatase and N–TiO2, as well as an efficient separation of photoinduced charges in the Ni,N–TiO2 samples. Moreover, they concluded that efficient TiO2-based photocatalysts with high Vis-light activity can be obtained by doping two or more elements. Except from the improved photoactivity during degradation of organic contaminants under visible light irradiation, the Ni-modified titania was found to exhibit excellent anti-microbial activity [24], [25], which could be an additional advantage of the Ni,N–TiO2. The presented above overview of the current state of knowledge in the area of nickel modified photocatalysts revealed high potential of such a modification. All the above mentioned works are focused, however, on Ni-modification of amorphous titania, pure anatase or anatase/rutile photocatalysts. In the present work we propose an efficient method of preparation of photoactive nickel, nitrogen co-modified rutile-TiO2.

The Ni,N co-modified rutile was prepared via impregnation with different Ni:TiO2 ratios (1–10 wt.%) followed by calcination at 1073 K in ammonia atmosphere. Additionally, single nickel-modified photocatalysts were prepared by applying argon atmosphere during calcination step. The obtained results provided us with variety of effects or structural, lattice and surface phenomena which allowed us to study in details the influence of nickel and nitrogen co-modification on rutile physicochemical properties and their effect on its photocatalytic activity. Our main goal was to prove that rutile-TiO2 can be successfully applied for photocatalytic application under all kind of irradiation after its proper modification; moreover, we have made an effort to explain the nature of the Ni,N co-modified rutile TiO2 photocatalytic activity.

Section snippets

Materials

Water suspension of an industrial grade amorphous titanium dioxide (TiO2/A) from sulfate technology supplied by “Chemical Factory Police S.A.” (Poland) was used as a starting material for the synthesis of N-modified, Ni-modified and (Ni,N)-co-modified rutile TiO2 photocatalysts. The BET surface area of the TiO2/A was 238 m2/g. Ni(NO3)2 × 9H2O was used as a source of nickel and gaseous ammonia (NH3) (Messer, 99.85%) was used as a source of nitrogen during the preparation process. Acetic acid was

The chemical composition and structure of photocatalysts

The actual concentration of nickel incorporated into the samples was evaluated by means of ICP–AES measurements and the results showed that about 80% of nickel used during impregnation process was successfully incorporated in the photocatalysts’ structure (Table 1).

In case of all samples the XRD analysis confirmed, as expected, the complete transformation of both amorphous titanium oxide and anatase into rutile. The crystallite sizes of rutile TiO2 exceeded 100 nm for all materials and therefore

Conclusions

In summary, nickel modification as well as nickel, nitrogen co-modification of rutile TiO2 result in obtaining highly active photocatalytic materials. Impregnation followed by calcination seems to be a promising method of preparation of this kind of materials. It was proved that charge transfer from TiO2 conduction band to NiTiO3 conduction band occurs in case of single-modified rutile and highly improves charge separation and thus, the photocatalytic performance of these samples under

Acknowledgments

This work was supported by National Centre for Science and Ministry of Science and Higher Education of Poland under project No. MNiSW/DPN/4878/TD/2010. The authors would like to thank professor Barbara Grzmil from West Pomeranian University of Technology, Szczecin for helpful assistance during XRD measurements.

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